I have a similar condition and it's said to be caused by stress and anxiety. Have a look at Conversion Disorder (specifically Hysterical Aphonia/Fluctuating Dysphonia). I realise this post is a few months old so hopefully you're better by now but mine has been going on for 18 months. If you are still ill, try psychotherapy, anti-anxiety tablets, antidepressants and relaxation techniques such as meditation. I don't know if they'll work for you but that's what I was told.